Hops are the female flowers of the hop plant Humulus lupulus. They are used as an ingredient in beer, to which they impart a bitter, tangy flavour. Hops are usually dried in an oast house before they are used in the brewing process. In the brewing process the wort (sugar-rich liquid produced from malt) is boiled with hops before it is cooled down and yeast is added, to start fermentation.
Boiling the wort with hops results in the extraction of alpha acids (humulones, such as humulone, adhumulone, cohumulone, posthumulone and prehumulone) into the sweet wort, which under the influence of temperature (thermal isomerisation) are partially isomerised to the corresponding iso-alpha acids (isohumulones). These iso-alpha acids are responsible for the characteristic bitter taste of hopped beer. Typical alpha acid levels in the wort at the start of the wort boiling are below 0.01 wt % (below 100 ppm). Disadvantages of this traditional hopping approach to impart bitterness are the inefficient alpha acid extraction and isomerisation, typically resulting in iso-alpha acid yields below 40%.
Hop utilization can be improved significantly by performing the alpha acid isomerisation outside the brewing process. To this end pre-isomerised hop products have been developed. These pre-isomerised hop product are produced starting from hop extract.
Hop extracts are usually obtained via liquid or supercritical carbon dioxide extraction. Carbon dioxide hop extracts provide predominantly alpha acids (humulones) next to beta acids (lupulones), and they can be further fractionated to obtain alpha acid enriched hop extracts.
Starting from these alpha acid containing extracts, the “off-line” pre-isomerisation of alpha acids outside the brewing (wort boiling) process can be achieved using alkali metal and/or alkaline earth metal based compounds via two approaches. The first approach applies a solvent-free reaction medium while in the other approach the transformation is performed after addition of solvents, either pure water or alternatively water mixed with an organic solvent.
It is known to isomerise alpha acid to iso-alpha acids by means of photolysis. AU 7540281, for instance, describes a process for the up-grading of the bitter substances from hops, and more particularly of practically pure alpha- and beta acids obtained from a highly purified hop extract, particularly by treatment with liquid carbon dioxide, characterized in that the alpha and/or beta acids are subjected to photolysis by sunlight, for a predetermined duration, under conditions adapted to produce an optimum quantic yield under suitable temperature conditions, preferably close to 60° C., to obtain respectively iso-alpha acids and deoxy-alpha acids
U.S. Pat. No. 4,767,640 describes a method of preparing light stable hop product that essentially consists of reduced isohumulones, and less than 0.5% unreduced isohumulones and non-isohumulones unstable products. Example 1 describes a process in which an isomerized hop extract was reduced, followed by acidification and separation of the aqueous layer and the oily layer. The oily extract so obtained contained 70% reduced isohumulone, 6.5% background material and less than 0.5% unreduced isohumulone. Example 3 describes how the oily extract was further purified to yield a “Post Purified Aqueous Phase” and a “Post Purified Oil Phase”. Example 4 describes the chromatographic separation of the oily extract of Example 1 into three fractions. Example 5 describes an experiment in which the light stability of the aforementioned purified extracts (“Post Purified Aqueous Phase” and a “Post Purified Oil Phase”) and of the three chromatographic fractions was evaluated. This was done by adding these materials to beer and irradiating for 2 hours with 600 foot candles of fluorescent light.
WO 93/02177 describes a process for producing a foam stabilizing and bittering agent for a malt beverage characterized by extracting hops under conditions which favour the separation and recovery of a fraction rich in adprehumulone, and subjecting the adprehumulone fraction to photo-isomerisation to produce iso-adprehumulone.
It is well-known that the flavour quality of beer is compromised by exposure to light, a phenomenon which is generally referred to as ‘lightstruck’ or ‘sunstruck’ flavour, and which is triggered by light-induced degradation of iso-alpha acids.
The formal mechanism for formation of beer lightstruck flavour in model systems, composed of isohumulones, riboflavin, and cysteine, on exposure to visible light, has been suggested by Kuroiwa et al. already in 1963. Photo-excited riboflavin induces cleavage of isohumulones to a 4-methylpent-3-enoyl radical, which undergoes decarbonylation to a 3-methylbut-2-enyl radical. Trapping of this stabilized allyl radical by a thiol radical derived from cysteine leads to formation of 3-methylbut-2-ene-1-thiol (3-MBT), the substance responsible for lightstruck flavour. 3-MBT has an extremely low flavour threshold level of around or below 1 ppt.
In order for the formation of 3-MBT to occur in beer besides iso-alpha acids, the reaction requires light energy in the 300-550 nm range of the spectrum, a photosensitizer (e.g. riboflavin, i.e. vitamin B2), and a sulphur source (e.g. sulphur containing amino acid).
The photolytic degradation of iso-alpha acids occurs as a consequence of the presence of an iso-3-hexenoyl side chain in the iso-alpha acid molecules. By modifying the molecular structure of the iso-alpha acids, notably by reducing the C═C and/or C═O bonds in the iso-3-hexenoyl side chain, substantial 3-MBT formation in beer can be prevented.
Reduced iso-alpha acid derivatives that are commercially available contain dihydro-, tetrahydro- and/or hexahydro-iso-alpha acids, and are usually added after the primary fermentation stage of the brewing process. The dihydro-iso-alpha acids (also called rho-iso-alpha acids) are obtained by the reduction of the carbonyl group in the aforementioned iso-3-hexenoyl chain to a hydroxyl group, generally using alkali metal borohydride as the reducing agent. The tetrahydro-iso-alpha acids are obtained via hydrogenation of the C═C bonds in the aforementioned iso-3-hexenoyl side chain and the isopentenyl side chain. The hexahydro-iso-alpha acids are produced by combining the aforementioned reduction and hydrogenation processes.
There is a need for a pre-isomerised hop extract that is light-stable, i.e. that does not substantially contribute to the formation of 3-MBT in light-exposed beer, but that has not been chemically reduced/hydrogenated.